The Return of the King, LoTR Appendix A, Annals of the Kings and Rulers: Númenor

Many treasures and great heirlooms of virtue and wonder the Exiles had brought from Númenor; and of these the most renowned were the Seven Stones and the White Tree. ... The ... Stones were divided.

Three Elendil took, and his sons each two. Those of Elendil were set in towers upon Emyn Beraid , and upon Amon Sûl, and in the city of Annúminas. But those of his sons were at Minas Ithil and Minas Anor, and at Orthanc and in Osgiliath. Now these Stones had this virtue that those who looked therein might perceive in them things far off, whether in place or in time. For the most part they revealed only things near to another kindred Stone, for the Stones each called to each; but those who possessed great strength of will and of mind might learn to direct their gaze whither they would. Thus the Númenóreans were aware of many things that their enemies wished to conceal, and little escaped their vigilance in the days of their might.

It is said that the towers of Emyn Beraid were not built indeed by the Exiles of Númenor, but were raised by Gil-galad for Elendil, his friend; and the Seeing Stone of Emyn Beraid was set in Elostirion, the tallest of the towers. Thither Elendil would repair, and thence he would gaze out over the sundering seas, when the yearning of exile was upon him; and it is believed that thus he would at whiles see far away even the Tower of Avallónë upon Eressëa, where the Masterstone abode, and yet abides. These stones were gifts of the Eldar to Amandil, father of Elendil, for the comfort of the Faithful of Númenor in their dark days, when the Elves might come no longer to that land under the shadow of Sauron. They were called the Palantíri, those that watch from afar; but all those that were brought to Middle-earth long ago were lost.

The Silmarillion, Of the Rings of Power and the Third Age

'What did the Men of old use them for?' asked Pippin, delighted and astonished at getting answers to so many questions, and wondering how long it would last.

'To see far off, and to converse in thought with one another,' said Gandalf. 'In that way they long guarded and united the realm of Gondor. They set up Stones at Minas Anor, and at Minas Ithil, and at Orthanc in the ring of Isengard. The chief and master of these was under the Dome of the Stars at Osgiliath before its ruin. The three others were far away in the North. In the house of Elrond it is told that they were at Annúminas, and Amon Sûl, and Elendil's Stone was on the Tower Hills that look towards Mithlond in the Gulf of Lune where the grey ships lie.